1. Field of the Invention
This invention relates to the field of synthetic cables. More specifically, the invention comprises devices for attaching standard cable rigging fixtures to an end of a synthetic cable using swaging techniques.
2. Description of the Related Art
Devices for mounting a termination on the end of a wire, rope, or cable are disclosed in detail in copending U.S. Application Ser. No. 60/404,973 to Campbell, which is incorporated herein by reference.
The individual components of a cable are known by various terms, such as “fibers,” “wires,” or “strands.” The nomenclature used often depends on the material used—such as steel “wire,” hemp “fibers,” or Kevlar “strands.” This invention disclosed herein can be applied to virtually any type of cable assembly, though its main application is for use with synthetic cables. For purposes of uniformity, the individual components of the cables described will be referred to as “strands” throughout this application.
Some type of fitting must typically be added to a cable in order to transmit a load to the cable. An old example of this idea is to wrap one end of a cable back upon itself—usually around an “eye” or “thimble” device—then clamp the cable to itself with one or more U-bolts. The resulting assembly on the end of the cable is referred to as a “termination.”
A more sophisticated approach is to provide a specialized fitting which can be deformed to lock it onto the end of a cable. This approach has traditionally been used with wire ropes. FIG. 1 shows a prior art wire rope 14. Standard rigging fixture 16 includes a socket 20 encompassed by a sleeve. The external surface of this sleeve is denoted as swaging surface 22. Some type of attachment feature is typically included—in this case eye 19.
Standard rigging fixture 16 is designed to be attached by a swaging operation. Those skilled in the art will know that the term “swaging” generally refers to plastically deforming one part over another part in order to lock them together. One example is swaging an automotive battery terminal onto the end of an electrical cable. A similar operation is carried out with the assembly of FIG. 1. Wire rope 14 is inserted into socket 20. Swaging surface 22 is then compressed in a swaging die.
FIG. 2 shows the assembly after the swaging operation. The diameter of the swaging surface has been plastically reduced in swaged region 24. The material has actually flowed plastically, meaning that the overall length is increased and the diameter reduced. The material is also compressed into the cavities between the wire rope strands, creating a mechanical interlock.
Swaging is often done with two piece or three piece dies. A deformed ridge may be created between the dies. A second swaging operation is done to eliminate this ridge. For a two-piece die, the first result is rotated about 90 degrees and re-swaged. For a three-piece die, the first result is rotated about 60 degrees and re-swaged. Three or more swaging steps may be needed to create a smooth and uniform exterior (if that it desired).
The result is the assembly shown in FIG. 2. The arrows depict the compressive forces of the swaging die, which has produced the deformed state shown. Those skilled in the art will know that swaging produces a very secure and predictable attachment. It is a mature technology for which processes and tools are widely understood and available. Thus, swaging is a very popular approach to attaching fixtures to wire rope.
There are many rigging fixtures designed for swaging. FIG. 3 shows a few of these rigging fixtures. All incorporate a swaging surface 22. From left to right, FIG. 3 depicts an eye fixture, a threaded stud fixture, a hook fixture, and a T-hook ball fixture. All of these can be attached to a wire rope by the previously-described swaging operation. This fact gives the cable manufacturer great flexibility. A stock of wire rope and standard rigging fixtures can be maintained. When an order is placed, the manufacturer cuts the cable to length and swages on the desired rigging fixtures.
These rigging fixtures are largely standardized. As they are manufactured in large volumes, they are relatively inexpensive. Thus, the traditional field of wire rope manufacturing enjoys a considerable economy of scale.
The field of synthetic cables has not traditionally enjoyed this flexibility or economy, since the prior art approach for creating swaged terminations does not work well for synthetic cables. Synthetic cables have very fine strands. The strands also tend to be quite slick. Very high swaging pressures are therefore required to produce an acceptable joint. These high pressures tend to kink and distort the individual strands, since synthetic strands tend to have poor compressive resistance. Thus, the techniques described in FIGS. 1 and 2 are ill suited for use with synthetic cables.
FIG. 4 shows a termination which is typical for synthetic cables. Termination 12 is affixed to the end of a synthetic cable (shown sectioned in half for visual clarity). Anchor 18 is attached to the strands of the synthetic cable by a variety of techniques. The version shown is attached by potting a length of strands into an expanding central cavity through the anchor.
The term “potting”—which is well known to those skilled in the art—can be described as follows: The cable strands are typically splayed into a diverging pattern and infused with liquid potting compound (using a variety of known techniques). The liquid potting compound is any substance which transitions from a liquid to a solid over time. The most common example would be a cross-linking adhesive such as an epoxy. Those skilled in the art know that such adhesives use two separate liquids which cross-link when mixed together. Such a liquid is mixed just prior to wetting the strands.
The wetted strands are at some point placed in a cavity within the anchor (in some cases prior to wetting and in some cases after wetting), so that when the liquid potting compound hardens the strands will be locked to the anchor. The anchor and the portion of cable locked therein are then collectively referred to as a termination.
Potted region 44 consists of strands locked within the solidified potting compound. The anchor is thereby attached to the cable. The anchor has a first end which is approximately even with the end of the cable and a second end proximate to the cable but distal to the end of the cable (the lower end in the orientation shown in the view).
The termination of FIG. 4 is an effective attachment of an anchor to a synthetic cable. FIG. 5 shows completed termination 12 alongside a standard rigging fixture 16. The reader will observe that the termination is not configured to be attached to the standard rigging fixture. Thus, the hardware traditionally used for terminating a synthetic cable is ill-suited for use with standard rigging fixtures traditionally used for terminating wire rope.